A radiation frequency identification (RFID) system comprises a RFID reader and a RFID tag. Data contained in the RFID tag is transmitted via radio frequency signals to the RFID reader. Power is transmitted, also via the radio frequency signals, to the RFID tag so that the RFID tag can operate without a built-in battery-based power source.
The reliability of an RFID system is affected by antennas of the RFID tag and the RFID reader. An electrical current flowing through the antenna generates an electromagnetic pulse that radiates outward with the speed of light. A direction of the radiation electric field (referred to as direction of polarization) is normal to the propagation direction of wave front of the electromagnetic wave. The radiation electric field is only affected by two parameters, one being the distance, wherein the magnitude of the radiation electric field is in inverse proportion to the distance, and the other being direction or orientation angle, wherein a relationship between the angle and the radiation electric field is referred to as an antenna field pattern. The magnitude of the radiation electric field varies with the direction or orientation angle of the antenna field pattern. This causes a problems associated with directionality in signal transmission between the RFID reader and a RFID tag. Further, once the antenna of the RFID tag is orientated orthogonal to the antenna of the RFID reader, signal reception or power reception may be not available. In a practical application, the location of the RFID tag may be constantly changed, and so is the orientation thereof. This often leads to a problem associated with the included angle as discussed above, making the performance of RFID tag reading extremely poor.
U.S. Pat. No. 7,042,413 provides a RFID tag that is made of a flat stock to form a three-dimensional antenna array. The tag has a first dipole antenna and a second dipole antenna, which are not parallel to each other and do not intersect each other so as to define a plane. A third dipole antenna is set in an outward projecting direction of the plane. Although this US patent forms a tag by cutting a flat stock to provide a three-dimensional antenna array, which eliminates the angle problem of signal reception by the tag, yet altering the antenna arrangement of the tag does not overcome the problem caused by the distribution of the radiation electric field in different directions or orientation angles of the antenna field pattern of the antenna of the reader and signal transmission is certain directions may still be not available.